Dataset: Houston Galveston Bay CO2 Flux

Field sampling

Galveston Bay is a semi-enclosed microtidal estuary located in the nwGOM [42]. With an average water depth of 3 m and surface area covering 1554 km2, Galveston Bay is the seventh largest estuary in the U.S. and the second largest estuary on the Texas coast [35, 43, 44]. Galveston Bay receives freshwater from the Trinity River, San Jacinto River, Clear Creek, and smaller bayous and creeks, with the Trinity River providing 70% of the freshwater entering the Bay [35, 45, 43, 44]. The Bolivar Peninsula and Galveston Island separate Galveston Bay from the GOM, with exchange of water between the Bay and the GOM occurring through Bolivar Roads, i.e., the mouth of the Bay [46, 43]. 

Monthly cruises were conducted between October 2017 and September 2018 on board the R/V Trident. Timing of the study allowed for examination of the factors regulating CO2 flux over the course of a year following Hurricane Harvey in late August of 2017. Although the study began more than 45 days (the residence time of the Bay) after Harvey, salinity recovery of the Bay was likely still ongoing in the inner and middle sections of the Bay [47, 48]. 

During each monthly survey, a transect was run between five water sampling stations, extending northwest from the Bay mouth (Station 1) opening to the Five Mile Marker on the Houston Ship Channel (Station 5). One offshore cruise in the nwGOM outside Galveston Bay was conducted in October of 2018. Underway pCO2 measurements were taken along a northwesterly transect extending from stations 1 through 5. A SUPER-CO2 System equipped with a LI-COR® LI-840A infrared gas analyzer was used to collect both water and air xCO2 after drying through a Peltier thermoelectric device­, and the xCO2 data after removing residual water vapor [49] was converted to pCO2 at sea surface temperature assuming 100% water vapor pressure [50]. Underway seawater was taken from a steel pipe attached to the side of the research vessel as it did not have a dedicated water intake system, and a diaphragm water pump was used to feed water to the equilibrator. In situ sea surface temperature and salinity were measured with a SeaBird Scientific SBE45® Thermosalinograph that was mounted parallel to the equilibrator of the SUPER-CO2 System. Prior to and following each sampling trip, the SUPER-CO2 System was calibrated using standards of known CO2 concentrations (273.3, 774.3, and 1468.7 ppm).

To calculate the pCO2 of seawater and air from measurements, the measured mole fraction of CO2 in seawater (xCO2, water) and measured equilibrator barometric pressure and xH2O were first used to calculate xCO2 in dry air (xCO2, air). This xCO2, air was then converted to pCO2 of equilibration (pCO2, eq) using measured temperature of equilibration (Teq) and water vapor pressure of equilibration, which was calculated from salinity and Teq according to methods outlined in [51]. Next, sea surface temperature (SST) and Teq were used to convert pCO2, eq to pCO2, water according to [52]. For pCO2, air, xCO2, air was converted to pCO2, air using water vapor pressure at SST and salinity, assuming 100% humidity [51].

Meteorological data

Three National Oceanic and Atmospheric Administration (NOAA) buoys from throughout Galveston Bay [60] provided six-minute interval averages of continuous wind speed data. The average wind speed for all three buoys during sampling times was calculated and applied to timing of sampling in Galveston Bay. Prior to calculations, wind speeds were converted to a height of 10 m (u10) using the wind profile power law [61], 

u1/u2 = (z1/z2)^P

where u2 is wind speed at height z2 = 10 m, u1 is the collected wind speed data at height z1, and the exponent P (0.11) around GOM area is extracted by [61]. 

United States Geological Survey [62] streamgages for the Trinity River (gage #08066500) and San Jacinto River, east fork (SJE; gage #08070200) and west fork (SJW; gage #08068000) were used to obtain freshwater discharge. These stations were identified as the closest gages to the mouths of the rivers having complete discharge data for the period of study. Discharges of less than or equal to 45 days (residence time of the Bay) prior to flux estimates were utilized [43, 44]. The Texas Commission on Environmental Quality (TCEQ) performs routine water quality monitoring, and TCEQ water sampling stations were used for river endmember values from the San Jacinto (average of west fork station #11243 and east fork station #11238) and Trinity (station # 10896) rivers [63]. River endmember DIC was calculated from TA and pH measurements using K1 and K2 constants from Millero [64], and pH value on the NBS scale. Seasonally weighted averages were calculated by summing the TA or DIC concentration multiplied by daily discharge values for all river measurements of that season and dividing by the sum of all discharge values for all river measurements of that season (using meteorological seasons). 

Historical data

Results from this study were compared to historical data for Galveston Bay obtained from the Surface Ocean CO2Atlas (SOCAT) database [80], which provided fCO2, water and xCO2, air values, along with surface seawater salinity, temperature, and depth, with observations from 2006 and 2010 through 2016, primarily during the month of September. SOCAT transects followed a similar route to our study transect, beginning near to station 4 and continuing outward into the GOM, with a side transect through the Galveston Channel, which separates Pelican Island from Galveston Island. fCO2 values were converted to pCO2 using the R package seacarb [81]. SOCAT data were analyzed independently from the results of this study.  As done before with ship data, SOCAT xCO2, air was converted to pCO2, air by accounting for water vapor pressure based on SST and SSS, assuming 100% humidity [74].